1. Field of the Invention
This invention relates to high speed fluid cutting jets, and more particularly to high speed slurry jets that use fluid-entrained abrasive particles to cut materials.
2. Description of Related Art
High speed fluid jets ("cutting jets") play an increasingly important role as a tool for cutting a variety of materials. In a cutting jet, a fluid, such as water or gas, entrains abrasive particles to form a slurry which is sprayed from an orifice of a nozzle at very high speeds (typically 100-500 m/sec). Like laser cutting devices, cutting jets are accurate, easily managed, and cause very little loss of material. However, abrasive jet cutting does not involve the high temperatures characteristic of laser cutting, and as a result are suitable for cutting practically any material. Further, the control system required for cutting jets is simpler and much cheaper than for laser cutting systems. Consequently, cutting jets can be used in a broad range of industries, from small machine shops and quarries to the large scale cutting requirements of the automotive and aircraft industries.
The most troublesome difficulty associated with cutting jets is wear of the nozzles, which presently limits their usefulness. Even using very hard materials, the high speed of the fluid, along with a particle size that can be as high as 40% of the nozzle diameter, can rapidly destroy a nozzle. Further, as the nozzle erodes, its kerf, or width of cut, changes, as does the dispersion of the fluid upon exiting from the jet nozzle. Consequently, nozzles must be replaced frequently, resulting in constant maintenance and inspection, loss of accuracy, and machine down time, all of which add to the cost of using a cutting jet.
Present attempts to solve this wear problem include seeding a pure liquid jet with abrasive particles only downstream of the nozzle, use of nozzles made of very hard materials (such as diamonds), using abrasive particles that are softer than the nozzle walls, and attempting to modify the flow structure of the nozzle in order to keep abrasive particles away from the nozzle wall. All of the presently available techniques have major deficiencies. Seeding downstream of the jet reduces the speed of the abrasive particles, and causes considerable expansion, scattering, and unsteadiness of the fluid flow. Diamond nozzles are expensive and almost impossible to form into desirable shapes. Use of abrasive particles softer than the nozzle reduces cutting efficiency. Modification to the jet flow structure by introducing secondary swirling flows near the nozzle walls is useful only with relatively slow flows and small abrasive particles; such modification also causes jet expansion and secondary flow phenomena that limit the capability to control the process.
Accordingly, it would be desirable to have an improved nozzle that overcomes the limitations of the prior art. The present invention provides such an improvement.